Conformal coatings provide a protective covering over automobile, aerospace and military electronic printed circuit boards. These coatings protect sensitive electronic components from corrosion of solder joints, fluids, hydraulic fluids, dirt, dust, moisture, mildew, physical abrasion or damage from handling and short circuits. Coated boards therefore can be protected from environmental, mechanical and electrical interferences.
The conformal coatings of the prior art have utilized chemistries such as acrylic, polyurethane, silicone, polyimide, epoxies, and parylene. These formulations, however, have suffered from several disadvantages. For example, conformal coatings formed from polyurethanes, acrylics, epoxy and silicone are two part systems which must be mixed prior to application and require continuous monitoring and solvent additions to control viscosity. These formulations usually also require long drying/curing times and release large amounts of volatile organic compounds (VOC) during curing.
Conformal coating systems based on acrylics are excellent from a production standpoint or brushing. However, acrylic coatings typically are formed by solvent evaporation which generates large amounts of VOC. Conventional acrylic coatings also are soluble in chlorinated solvents such as tricholorethane or methylene chloride.
Conformal coatings based on polyurethanes are available as either single or two-component systems. Polyurethane coatings offer excellent humidity and chemical resistance and good dielectric properties. Single-component urethanes are relatively easy to apply and exhibit relatively long working pot life. However, single-component polyurethanes typically require a curing time of three to ten days at room temperature to reach optimum physical characteristics. Two-component polyurethanes typically achieve optimum cure at elevated temperatures within one to three hours, but exhibit relatively short working pot life.
Surface preparation of substrates prior to application of polyurethane based coatings is also important, since even minute quantities of moisture on a substrate board could produce blistering under humid conditions. Blisters, in turn, may lead to electrical failures and mandate costly rework. Polyurethane coatings are insoluble in most common solvents, which is a drawback to rework. Thus, replacement of a component on a polyurethane coated board requires a corrosive stripper to remove effectively all traces of the polyurethane film. However, extreme caution must be exercised when such a stripper is used, because the stripper also may corrode metallic surfaces on the board.
Epoxy resin systems also have been employed for conformal coating of printed circuit boards. Epoxy resins are available as two component systems only. Epoxy resin coatings provide good humidity resistance and high abrasive and chemical resistance. However, epoxy resins are virtually impossible to remove chemically for rework because any stripper that will attack the coating also will attack the epoxy-glass of the printed circuit board as well. Thus, the only effective way to repair an epoxy resin coated board is to burn through the epoxy coating with a hot knife or soldering iron. However, burning introduces a cosmetic defect which is unacceptable to many consumers. Moreover, epoxy resins shrink somewhat during cure. Accordingly, a buffer material must be placed around fragile electronic components to prevent fracturing from shrinkage. Curing of epoxy systems can be accomplished in one to three hours at elevated temperature, or four to seven days at room temperature. Epoxy resins exhibit a relatively short working pot life which is an additional disadvantage.
Silicone resins have been employed for conformal coatings. Silicone resin coatings provide high humidity and corrosion resistance along with high temperature resistance which makes silicone resins preferred for coating printed circuit assemblies that contain high heat-dissipating components such as power resistors. However, silicone resins are relatively thick and therefore difficult to apply. Moreover, silicone resins require a relatively long cure time, and repairability is difficult. The only effective way to repair a silicone resin coated circuit board is to mechanically remove the coating.
The prior art has employed polyimides for conformal coating circuit boards. Polyimide coatings provide high-temperature, moisture and chemical resistance over extended periods of time. However, polyimide coatings require high temperature cure (one to three hours at 200.degree. to 250.degree. C. which could damage heat sensitive electronic components. Also, since polyimides are high-temperature, moisture and chemical resistant, the only effective way to repair a polyimide coated board is to mechanically remove the coating.
Several of these disadvantages have been addressed by use of ultraviolet (UV) curable conformal coatings. The UV curable coatings of the art are one part systems which usually are devoid of solvents to thereby reduce or eliminate the (VOC) emission. These systems are cured rapidly by UV to provide tack-free coatings via free-radical or cationic polymerization. This enables immediate handling of the coated articles for further processing, storage or shipping. Moreover, use of UV curable coatings reduces overall processing time and energy costs as compared to thermally cured coatings.
The UV cured systems of the art have been useful for coating flat surfaces. However, printed circuit boards which bear electronic components tend to be oddly configured. These odd configurations cause shadow areas which cannot be reached by uv radiation. The coated portions in these shadow areas therefore remain wet and cannot be handled immediately. Secondary cure mechanisms such as heat have been employed to polymerize these shadowed areas. The drawback of heat induced secondary curing, however, is that temperatures of up to 100.degree. C. are required to cure the shadowed areas. These temperatures can adversely affect sensitive electronic components.
A need therefore exists for conformal coatings which overcomes the above drawbacks. We therefore have developed a novel, dual-curing conformal coating composition that is a solvent-free low viscosity liquid that rapidly cures into a tack-free polymer when exposed to UV light. Those portions of the coatings which are not cured by UV can be cured in less than 24 hours by a moisture cure component provided in the coating formulation.